Device for the optical copying of photographic negatives



Feb. 13, 1962 A R ETAL 3,020,801

DEVICE FOR THE OPTICAL COPYING OF PHOTOGRAPHIC NEGATIVES Original FiledMarch 30, 1954 4 Sheets-Sheet 1 INVENTOR5 fr! flmdl M BY WM X M Feb. 13,1962 F. LANDER ETAL 3,020,801

DEVICE FOR THE OPTICAL COPYING OF PHOTOGRAPHIC NEGATIVES Original FiledMarch 50, 1954 4 Sheets-Sheet 2 INVENTORS r-M J M/- M Feb. 13, 1962 F.LANDER ETAL 3,020,301

DEVICE FOR THE OPTICAL COPYING 0F PHOTOGRAPHIC NEGATIVES Original FiledMarch 30. 1954 4 Sheets-Sheet 3 Feb. 13, 1962 LANDER ETAL 3,020,801

DEVICE FOR THE OPTICAL COPYING 0F PHOTOGRAPHIC NEGATIVES Original FiledMarch 30, 1954 4 Sheets-Sheet 4 hi4 fflak? jffm n7 United States Patent0, DEVICE FOR THE OPTICAL COPYING 0F PHOTOGRAPHIC NEGATIVES Fred Lander,Bahnhofstrasse 49, Wil, St. Gall, Switzerland, Max Lattmann, Zurich, andRobert Oberacker, Neuchatel, Switzerland; said Lattmann and Oberackerassignors to said Lander Continuation of application Ser. No. 419,872,Mar. 30, 1954. This application Oct. 7, 1958, Ser. No. 765,851 Claimspriority, application Switzerland Apr. 1, 1953 21 Claims. (Cl. 8824) Theinvention relates to adevice for the optical copying of photographicnegatives, and more particularly for the enlarging thereof.

The present application is a continuation of application Serial No.419,872, filed March 30, 1954, now abandoned.

a It is an object of the present invention to provide a device of thekind referred to and of small structural height which is accordinglysuitable for use on a table top.

It is another object of the invention to provide a device of the kindreferred to by means of which the time of exposure of the lightsensitive paper, on which the photographic negative is to be copied, isautomatically determined in accordance with the characteristics of thepaper and with the size of the image of the projected negative on thepaper.

It is yet another object of the invention to provide an instrument ofthe kind referred to which gives the operator an overriding influence onsaid automatically determined exposure.

It is still another object of the invention to provide an instrument ofthe kind referred to which permits focussing of the image of thenegative to be copied on the plane to be occupied by-the light sensitivepaper, without starting the automatic timing cycle.

With these and other objects in view we provide a device for he opticalcopying of photographic negatives, comprising incornbination: a casing,a light-permeable support for the light sensitive paper and an imageframe arranged on top of the said casing, a photo-electric cell arrangedwithin the said casing, a condenser electrically connected to the saidphoto-electric cell and charged by the photo-current thereof produced asa function of the quantity of light penetrating from an external lightsource of a photographic projector through the said light-sensitivepaper and light-permeable support thereof to the said photo electriccell, and a switching relay arrangement electrically connected to thesaid condenser and to the said external light source automaticallydischarging the said condenser upon its attaining a predeterminedvolt-age, and by the discharge current thereof switching off the saidexternal light source, the said casing, lightapermeable support, photoelectric cell, condenser and switching relay arrangement forming a structural unit with the said image frame.

Preferably the electrical switching elements and the said photo electriccell are arranged along the margins of the said casing,,in the center ofwhich a reflector is arranged which reflects the light penetrating fromthe said external-light source through the light sensitive paper I andlight-permeable support to the photo-electric cell.

These and ot-her'objects and features of our invention will be clearlyunderstood from the following description which, by way of example,illustrates an embodiment thereof, with reference to the accompanyingdrawings. in which: 1 a a t FIG. 1 shows a device of the invention inperspective top view,

3,020,801 Patented Feb. 13, 1 96 2 FIG. 2 is a view similar to FIG. 1with the "cover plate and parts of the margin portions broken oil.

FIG. 3 shows a simplified electriewiring diagram of the device of FIGS.1 and 2, I

FIG. 4 shows a complete electric wiring diagram of the device of FIGS. 1and 2. a

With reference to FIGS. 1 and 2, the device of the invention consists ofa relatively low casing 10,on which a lid 12 is tiltably mounted bymeans of hinges 11; In the cambered marginal portions 13 and 14 of thecover, slots 15 and 16 are provided for masks 17 and'18 which can beshifted after loosening of the fixing screws 19 and 20. Thereby thelength L and width B of an image frame 21' can be varied. That part ofthe lid 12 which remains free when the image frame is adjusted to itsmaximum possible size, forms a light-permeable cover plate, for examplea ground-glass screen 1, onflwhich a photopaper 2 is placed with thesensitive'l'ayer' up (see FIGS. 3, 4) of this sensitive layer an imageof a film to be enlarged is projected in a usual manner by an enlargerapparatus suspended on a support. Focussing is facilitated by a plate 22which is articulated by means of the hinge 23 on the mask 17, so that itcan be tilted into the image frame 21. That side of the focussin'g'plate 22, which is directed upward in the rest position illustrated infull lines, is black, whereas the other side is pure white. Afterfocussing, the plate 22 is folded back into the rest position and thephoto paper 2 is inserted, the size of the image frame desired beingdetermined by the position of the masks l7 and 18. A photo cell40 isbuilt into the casing 10 below an edge of the cover plate 1. Thephotocell 40, in conjunction with an aim plifier system, converts thelight entering through the image frame, the photo paper and the lightpermeable ground glass screen, into a charging current for, a condenser.According to the intensity of this current the condenser is charged upmore or less quickly to ap-r'edeterinined voltage, whereupon the lightsource ofthee'rilarger apparatus is switched off by a controlswitcherrangement, the resulting exposure period beingcontrolled by theimage luminosity and the sensitivity of the layer.

The sensitivity of the layer and the light permeability of thephotographic paper selected can be taken into account as an empirical incoefficient for the paper concerned. Since with a high sensitivity theperiodof exposure has to be reduced, andsince a high light-permeabilityincreases the charging current, these two properties are to be takeninto account in the same sense. The resulting paper coefiicient can beset on an adjustment button 24, and can be read from a scale 25. i i

The device is made ready for operation by 610 'ng the mainswitch 26, andit is thereby connected tothe alternating current mains through thecable 27." The connector cable 29 for the enlarger apparatus isconnected to the terminal sockets 28 while the connector cable 30 forthe illumination of the dark room is con nected to the pair of terminals31.

The'adjustrnent switch 32 permits the lightvso urce of the enlargerapparatus to be switched on without starting the device for theautomaticcontrol of exposure. The plate 22 is then pivoted about itshinge 23 into the ima e frame,[and theenlarger is focussed. Aswitch'handle33 which is held by springs in the middleposition'illustrat'ed, causeswhen displaced towards the plus-positionatria: crease of the exposure period as compared tothat dctermined bythe position of the other adjustment merribers; while turning the switch33 towards the minus position results in a shortening of the exposureperiod'so that the operator is given the] opportunity of varying; theexposure period at will. ,A-starter button 34, when temporarilydepressed, starts the device fortthe automatic control of the exposureperiod. The illumination of the dark room is switched off and the lightsource of the enlarger is switched on, the condenser is charged up andswitches the light source of the enlarger off after a certain period, aswill be described later, and at the same time again switches on theillumination of the dark room. The masks 17 and 18 are fixedly connectedwith sliding contacts 35 and 35' of voltage divider resistances 36 and.37 the exposure period is automatically adjusted to the correct valuefor each image size.

With reference to FIG. 3, the basic principle of operation of the devicewill now be explained. A photo multiplier tube 40 comprises a photocathode 41, a number of bafile electrodes 42 and a collector electrodeor anode 43. A source 44 for a rectangular alternating voltage, i.e.sinusoidal voltage with the amplitude peeks cut ofi, has one terminalconnected to the ground wire 45, and another terminal connected by theconductor 46 to one terminal of the secondary coil 47 of a tappedtransformer 48. The use of alternating voltage permits the use oftransformers for obtaining different voltages. The photo cathode 41 isconnected to the other terminal of coil 47, and each of the bafiieelectrodes 42 is connected with an intermediate tap of the coil 47. Theanode 43 of the photo multiplier tube is connected through a currentlimiting resistance 49 and a condenser 50 to the ground wire 45. Thevoltage of the source 44 is thus applied to the last of the baffleelectrodes and to the anode, the impedances of the resistance 49 and ofthe condenser 50 being negligibly small as compared with the internalresistance of the cell. The transformer 48 is energized by the primarycoil 51 which is connected to the source of voltage 44 through amultiple voltage divider arrangement. This voltage divider arrangementcomprises four voltage divider resistances or potentiometers 36, 37, 52and 53, the movable contact of the potentiometer 36 being coupled withthe mask 17 for adjustment of the width to the image frame, and themovable contact of the potentiometer 37 being coupled with mask 18 forthe adjustment to the length of the image frame. The movable contact ofthe potentiometer 52 is held in a center position by the springs 54, andcan be displaced from this center position by a handle 33. The movingcontact of the potentiometer 53 is coupled to the adjustment knob 24.When the apparatus is adjusted for an image size and the correctionhandle 33 is not moved, the voltage at the terminals of the coil 51, andaccordingly at the bafile electrodes and at the photo cathode depends onthe adjustment of the knob 24. The cathode is exposed to the light whichpenetrates through the photographic paper 2 and the light-permeableground glass screen 1 into the photo multiplier tube 40. This tube 40 isarranged under the stationary marginal portion 13 of the image frame 21in order to reduce the height of the casing as far as possible, as willbe seen in FIG. 2. In this manner, considerable structural height can besaved as compared with known arrangements, in which a photo electriccell is arranged centrally below the cover plate. All other operatingelements are also arranged along the margins of the casing. In orderthat the light, which falls through portions of the cover plate 1adjacent to the tube 40, and the light which falls through portions ofthe cover plate remote from the tube, may act on the photo cathode 41 tothe same extent, a black-and-white screen plate 56 which curves uptowards the cover plate and whose content in white increases withincreasing distance from the photo electric cell is arranged on thebottom of the device. In FIG. 2,- uneven distribution of black and whiteon the screen plate 56 is indicated by a corresponding distribution ofstripes. The same effect could alternatively be attained placing a gridplate under the ground glass screen 1. The grid plate would have to beso designed that in the neighborhood of the photo multiplier tube itweakens the passage of light, and that it favors the passage of light inregions more remote from the tube. 1

In those phases of the alternating voltage in which the photo cathode4-1 and the baffie electrodes 42 have a negative charge relative to theanode 43, current flows through the resistance 49 to one of the platesof the condenser 50 the other plate of which is connected to the groundwire 45. The intensity of this current for a given quantity of lightacting on the cathode 41 depends primarily on the voltage of the bafileelectrodes 42, i.e. on the voltage at the terminals of the coil 51. Anyshifting of the movable contact of the potentiometer 53 in the directionof the arrow reduces this voltage. The adjustment knob 24, when beingturned to a higher coeflicient of the photographic paper, adjusts theposition of the moving contact in the direction of this arrow. Anyadjustment of the moving contact of the potentiomemter 52 in oppositionto the restoring forces of the springs 54 in the direction of the arrowlikewise reduces the voltage on the coil 51, i.e. the charging currentfor a given quantity of light. The movable contacts of thepotentiometers 36 and 37, also reduce the input voltage for the coil 51when displaced in the direction of the arrows and thereby reduce thecharging current for a given light condition. The coupling of themovable contacts with the masks 17 and 18 is such that the anode currentis reduced when the image size is increased because in this case morelight falls on the cathode 41. According to the intensity of thecharging current, the condenser 51 is loaded more or less quickly to apredetermined voltage, and it should be noted that the relay contact 57is opened in a manner to be described later when the device is inoperation, said contact being shown in the rest position. One electrodeof a glow discharge tube 58 is connected to the anode side of thecondenser 5% whereas the other electrode is connected to the ground wire45 over the primary coil 59 of a pulse reversing transformer 60 and asource 61 of a negative bias voltage. When a predetermined voltage ofthe condenser 50 is reached, the glow discharge tube 58 strikes, and acurrent pulse flows through the coil 59. The control grid of thethyra-tron tube 64 is connected over the secondary coil 62 of thetransformer 60 and a source of negative bias voltage 63 to the groundwire. Its heating filament is heated from the source of voltage 65,while its cathode is connected to ground over the conductor 66. Theanode of the thyratron 64 is in circuit wtih the positive terminal of asource of voltage 71 over the exciter coil of a relay 67 and a contact68 of this relay as well as a conductor 70. This anode is also incircuit with one of the stationary contacts of a double-pole contact 69of this relay 67. The exciter coil of a further relay 7?. to whichbelong the contacts 57 and 73 is in circuit with the other stationarycontact of the aforesaid double-pole contact 69 and with a conductor 74,to which the contact 73 and one of the stationary contacts of the pushbutton switch 34 are connected. The positive terminal of the currentsource 71 is connected to the opposite contacts of the switches 73 and34. The exciter coil of a third relay 75 is connected to the positiveterminal of the current source 71 over the holding contact 73, and withthe ground wire 45 over contact 69. The two stationary contacts of astarting switch 34 are connected to the conductor 74 and to the positiveterminal of the'current source 71, respectively. When the pushbuttonswitch 34 is temporarily actuated, the coil of the relay 72 is thereforesupplied with current over the following circuit:

Positive pole of the current source 71-pushbutton switch 34conductor74-coil of relay 72--double-pole contact 69-ground.

By the excitation of the relay 75 its double-pole contact 76 isswitched, so that instead of the dark room lamp 78 the lamp 77 0f theenlarger is connected to the alternating current mains. The thyratronvalve 64 remains blocked for the time being, because its grid has anegative bias. When after a certain time the voltage on the condenser 50has reached a sufiiciently high value, the glow discharge tube 58strikes, whereby a current flows over the coil 59 and the battery 61 toground. The negative bias of the glow discharge tube 58 limits theeffect of manufacturing variations, which could cause a shifting of thestriking voltage by several volts, and corresponding variations of thecharging time required. The current pulse in the coil 59 induces in thecoil 62 a voltage pulse which makes the thyratron grid positive, so thatcurrent flows in the anode circuit thereof. The relay 67 is therebyexcited, and accordingly switches its contacts 68 and 69. By the openingof the contact 68 the anode of the thyratron is separated from thesource of voltage 71, so that the thyratron is extinguished. By theswitching of the contact 69 the holding circuits for the relays 72 and75 are interrupted so that they are deenergized. Thereby the holdingcontact 73 and the bridging contact 57 of the relay 72 drop back intotheir rest position illustrated, so that the condenser '59 is dischargedand the holding circuit for the relays 72 and 75 is interrupted. Thesubsequent deenergizing of the relay 67 can therefore neither cause arenewed igniting of the thyratron nor renewed excitation of the relays72 and 75. By the igniting of the thyr'atron, and by the deenergizing ofthe relay 75 effected thereby, the switch-over contact 76 of the latterdrops back into therest position shown, so that the light source 77 ofthe enlarger is again switched off, and the illumination 78 of the darkroom is switched on.

The described arrangement of the condenser 50 prevents in an effectivemanner the formation of by-paths for the anode current of the photoelectric cell 40. Merely the components enclosed in the chain-dottedframing have to be particularly well insulated, whereas for example thetransformer 48 does not open any by-paths for the charging current. Thelight source 77 of the en larger can be connected to the mains by theswitch 32 without starting the automatic exposure controls. This allowsfocussing of the image.

In the scheme according to FIG. 4 those elements of the wiring diagramwhich are constructed exactly like those in FIG. 3, are denoted by thesame reference numerals, while those which have an analogous functionare'referred to by a numeral increased by 100.

The various current sources are different from those shown in FIG. 3.The alternating currentrnains is connected over the feeder cable 27, thefuse 80, the main switch 26 and the resistance 81 to the primary coil ofthe transformer 82. Likewise, the primary coil 84 of the transformer 85is connected to the mains over the switch 26. A central tap of thesecondary coil 86 of the transformer 82 is connected to the ground wire45, and the two terminals of this coil 86 are connected over thecondoctors 87 and 88 with respective rectifiers 89 and 90. The otherterminals of these tworectifiers are connected to the ground wire 45over a stabilisor glow discharge tube 91. By the action of the currentlimiting resistance 81 and of the rectifiers 90, 89 an alternatingvoltage with cut-off peaks, i.e. substantially a rectangular alternatingvoltage,'is generated between the conductor, which is connected to thecoil 47, and the ground wire 45. For generating the input voltage forthe coil 51 as a function of four parameters, the four potentiometers136, 137, 152 and 158 are provided, which, in a circuit slightlydifferent from that according to FIG. 3, have the same functions as thevoltage dividers 36, 37, 52 and 53 of FIG. 3., An intermediate tap 86 ofthe coil 86 is connected to a ground over therectilier 92, and theparallel arrangement of a resistance 93 and a filter condenser 94, sothat thecircuit points 161 and 162 form volt-age sources the stabilisortube 91, another filter condenser a being arranged between the conductor70 and the ground wire 45. The secondary coil 95 of the transformer 85forms at the same time the current source for the heating filament ofthe thyratron and for the relays 172 and which in this embodiment arealternating current relays. The primary coil 96 of a transformer 97bridges over the resistance 81 and induces correcting voltages in thesecondary coils 98a, 98]) which voltages are supplied to thepotentiometers 136 and 137. Thereby the coil 51 is supplied with anaccurately rectangular voltage, because the peak voltages aretransmitted inversely to the voltage divider system at the resistance81. Otherwise the manner of operation of the arrangement illustrated inFIG. 4 conforms to that of the arrangement according to FIG. 3.Temporary actuation of the push button switch 34 excites the relays 172and 175, the dark room illumination 78 is switched off, and the lightsource 77 of the enlarger is switched on. At the same time the bridgingcontact 57 is opened, so that the voltage on the condenser 50 begins torise. When reaching a predetermined voltage value, the glow dischargetube 58 strikes, so that the thyratron 64 restores the originalcondition of the device by means of the relay 67. The characteristics ofthe voltage divider resistances 136, 137 are to be selected in such amanner that an increase of the image frame to twice its length, and alsoan increase of the image frame to twice its width have the same effectas an increase in sensitivity to twice the value. In order that thedevice may also be suitable for making enlargements in colourphotography, the condenser 50 is advantageously made exchangeable.

Under certain circumstances, depending on the kind of glow dischargetube 58 used, it may be necessary to connect the same to earth aftereach use of the device by means of additional circuit members, in orderthat the glow discharge lamp does not strike at different voltage valuesof the condenser during subsequent operations, for example owing toaccumulating static charges.

While we have herein described and illustrated in the accompanyingdrawings what may be considered a typical and particularly usefulembodiment of our invention, we Wish it to be understood that we do notlimit ourselves to the particular details and dimensions described andillustrated, for obvious modifications will suggest themselves to aperson skilled in the art.

We claim:

1. A device for the optical copying of photographic negatives,comprising, in combination: a casing, a light-perpassing throughdifferent portions of said support and.

transmitted to said photo electric cell in such a manner that thequantity of light transmitted to said photo electric cell issubstantially proportionate to the overall quantity of light passingthrough said light-permeable support into said casing, a condenserelectrically connected to the said photo electric cell and charged bythe photo current thereof produced as a function or" the quantity oflight penetrating from an external light source of a photographicprojector through the said light-sensitive paper and light-permeablesupport thereof to the said photo electric cell, and a switching relayarrangement electrically connected to the said condenser and to thesaid'external light source automatically discharging the said condenserupon its attaining a predetermined voltage, and by the discharge currentthereof switching off the said external light source, the said casing,light-permeable support, photo electric cell, condenser, and switchingrelay arrangement forming a structural unit with the said image frame.

2. A device for the optical copying of photographic negatives,comprising, in combination: a casing, a light permeable support for thelight sensitive paper and an image frame arranged on top of said casing,said lightpermeable support defining a light accessible area thereunderin said casing, a photo electric cell arranged within the said casingoutside said area; reflector means in said area for reflecting lightpassing through said light-permeable support onto said photo electriccell, said reflector means having a plurality of reflecting elements ofvarying reflectivity arranged in such a manner that the quantity oflight reflected onto said photo electric cell is substantiallyproportionate to the overall quantity of light passing through saidlight-permeable support into said casing, a condenser electricallyconnected to the said photo electric cell and charged by the photocurrent thereof produced as a function of the quantity of lightpenetrating from an external light source of a photographic projectorthrough the said light-sensitive paper and light-permeable supportthereof to the said photo electric cell, and a switching relayarrangement electrically connected to the said condenser and to the saidexternal light source automatically discharging the said condenser uponits attaining a predetermined voltage, and by the discharge currentthereof switching ofi the said external light source, the said casing,light-permeable support, photo electric cell, condenser, and switchingrelay arrangement forming a structural unit with the said image frame.

3. A device for the optical copying of photographic negatives,comprising, in combination: a casing, a light-permeable support for thelight-sensitive paper and an image frame arranged on top of said casing,said light-permeable support defining a light-accessible area thereunderin said casing, a photo electric cell arranged within the said casingand laterally offset from the center of said area; reflector means insaid area for reflecting light passing through said light-permeablesupport onto said photo electric cell in such a manner that the meanintensity of light reflected to said photo electric cell isproportionate to the mean of light intensities passing through differentportions of said light permeable support into said casing andirrespective of the distance of said different portions from said photoelectric cell, a condenser electrically connected to said photo electriccell and charged by the photo current thereof produced as a function ofthe quantity of light penetrating from an external light source of aphotographic projector through the said light-sensitive paper andlight-permeable support thereof to the said photo electric cell, and aswitching relay arrangement electrically connected to the said condenserand to the said external light source automatically discharging the saidcondenser upon its attaining a predetermined voltage, and by thedischarge current thereof switching off the said external light source,the said casing, light-permeable support, photo electric cell,condenser, and switching relay arrangement forming a structural unitwith the said image frame.

4. A device for the optical copying of photographic negatives,comprising, in combination: a casing, 21 lightpermeable support for thelight-sensitive paper and an image frame arranged on top of said casing,said lightpermeable support defining a light-accessible area thereunderin said casing, a photo electric cell arranged within the said casingoutside said area and laterally offset therefrom; curved reflector meansin said area for reflecting light passing through said light-permeablesupport onto said photo electric cell, said reflector means extendingarcuately from one area adjacent said light-permeable support and moreremote from said photo electric cell to another area nearer said photoelectric cell and remote from said light-permeable support and having aplurality of elements of varying reflectivity arranged in such a mannerthat the overall reflectivity of said reflector means decreases fromsaid one area towards said other area, whereby the quantity of lightpassing through said light-permeable support and received by said photoelectric cell is substantially proportionate to the overall quantity oflight passing through said light-permeable support into said casing, acondenser electrically connected to the said photo electric cell andcharged by the photo current thereof produced as a function of thequantity of light penetrating from an external light source of aphotographic projector through the said light-sensitive paper andlight-permeable support thereof to the said photo electric cell, and aswitching relay arrangement electrically connected to the said condenserand to the said external light source automatically discharging the saidcondenser upon its attaining a predetermined voltage, and by'thedischarge current thereof switching off the said external light source,the said casing, light-permeable support, photo electric cell,condenser, and switching relay arrangement forming a structural unitwith the said image frame.

5. Apparatus for the optical copying of photographic negativescomprising, in combination, a casing; a lightpermeable support for thecopying paper and an image frame arranged on top of said casing; a photoelectric cell arranged within said casing, said photo-electric cellincluding a photo cathode, a collector anode and a plurality of battleelectrodes providing electron amplification; a voltage source forapplying voltage to said baflle electrodes; adjustable electricalcontrol members connected in circuit with said bafile electrodes andsaid voltage source for controlling the voltage applied to said baffleelectrodes; a capacitor connected in circuit with said photo electriccell and charged by the current thereof produced as a function of thequantity of light passing through the copying paper from an externallight source to said photoelectric cell; a source of substantiallyrectangular alternating voltage connected in circuit with the last ofsaid baflie electrodes and one plate of said capacitor, the other plateof said capacitor being connected to said collector anode of saidphoto-electric cell; a feeder transformer having a secondary coil with aplurality of taps connected to the respective baffle electrodes, andhaving a primary coil connected to said source of substantiallyrectangular alternating voltage; and a switching relay arrangementconnected in circuit with said capacitor and the external light sourcefor automatically discharging said capacitor when the same reaches apredetermined voltage, the discharge current of said capacitor switchingoff the external light source, said casing, light-permeable support,photoelectric cell, capacitor and switching relay arrangement forming astructural unit with the image frame.

6. A device as claimed in claim 5, wherein the said source of asubstantially rectangular alternating voltage contains an alternatingcurrent transformer having a primary coil and a secondary coil, thelatter being electrically symmetrically disposed relative to mass, tworectifiers and a glow discharge valve, each end of the said secondarycoil being connected to mass over one of the said rectifier and the saidglow discharge valve in common to them, and the last of the said bafileelectrodes of the electronic amplifier of the said photo electric cellbeing connected to one of the said ends, the collector anode of the saidphoto electric cell being connected over the said condenser to mass, theprimary coil of the said transformer being connected to alternatingcurrent means, and moreover comprising a current limiting resistanceconnected in series with the said primary coil cutting oif the voltagepeaks and a multiple voltage divider arrangement connected to the saidfeeder transformer for the said photo electric cell, and providing acontrollable part-voltage thereto.

7. A device as claimed in claim 6, comprising in addition: a correctiontransformer connected to the said current limiting resistance andtransforming the voltage generated across the said resistance back intothe said voltage divider arrangement with inverted phase as similatingthereby the said substantially rectangular feedor voltage of the saidfeeder transformer for the said photo electric cell to a trulyrectangular alternating voltage. p

8. A device as claimed in claim 6, comprising in addition: an impulsereversing transformer and another glow discharge valve, the saidcollector anode of the said photo electric cell being connected over thesaid other glow discharge valve and over the primary coil of the saidimpulse reversing transformer to an external direct negative voltagesource.

9. A device as claimed in claim 8, comprising in addition: a thyratronvalve, a relay having a coil and switching contacts, a starting switch,the said thy-ratron valve having a control grid connected over thesecondary coil of the said impulse reversing transformer to an externaldirect negative voltage source, and an anode connected through the saidrelay coil to an external direct positive voltage source, the saidswitching relay arrangement being electrically connected to andenergized by the operation of the said starter switch and therebyswitching off an external illumination for the dark room and switchingon the said external light source of the photographic projector, thesaid relay extinguishing the said thyratron valve again after the samehas struck, and switching ofl' the said switching relay arrangement fromits external source of supply current.

1.0. Apparatus for the optical copying of photographic negativescomprising, in combination, a casing; a lightpermeable support for thecopying paper and an image frame arranged on top of said casing; aphoto-electric cell arranged within said casing; a capacitor connectedin circuit with said photo-electric cell and charged by the currentthereof produced as a function of the quantity of light passing throughthe copying paper from an external light source to said photo-electriccell; shiftable masks arranged on said casing for defining the lengthand width of said image frame; a switching relay arrangement connectedin circuit with said capacitor and the external light source forautomatically discharging said capacitor when the same reaches apredetermined voltage, the discharge current of said capacitor switchingoff the external light source; and adjustable electrical control membersrespectively mechanically coupled with each of said shiftable masks andconnected in circuit with said capacitor for counterbalancing any effectdue to an increased quantity of light passing through said photo-copyingpaper due to the adjustment of said masks to an increased size of imageframe, said casing, light-permeable support, photo-electric cell,capacitor and switching relay arrangement forming a structural unit withthe image frame.

11. Copying apparatus in accordance with claim wherein saidphoto-electric cell and the switching relay arrangement are arrangedalong the walls of the casing providing a chamber beneath the copyingpaper, a reflector arranged in said chamber for reflecting light passingthrough the copying paper towards the photo-electric cell, saidreflector including light attenuating means between the plane of thecopying paper and the photo-electric cell so that light which passesthrough the copying paper and is reflected by the reflector to thephoto-electric cell is attenuated an amount inversely proportional tothelength of the path traveled by the light.

12. Copying apparatus in accordance with claim 10 wherein thephotoelectric cell has an anode and is provided with built-in batheelectrodes for electron amplification, the anode circuit of said cellbeing formed as the charging current circuit for said capacitor, voltagevarying means connected to said electrical control members for changingthe voltage on said baflie electrodes. in accordance with thelight-sensitivity and light-permeability of the copying paper, saidcontrol members including two voltage varying members coupled with said,shiftablc masks and a third voltage varying member for independentlychanging the illumination time of the copying paper whenever desired. 7

13. Apparatus for the optical copying of photographic negativescomprising, in combination, a casing; a lightpermeable support for thecopying paper and an image frame arranged on top of said casing; aphoto-electric cell arranged within said casing; a capacitor connectedin circuit with said photo-electric cell and charged by the cur rentthereof produced as a function of the quantity of light passing throughthe copying paper from an external light source to said photo-electriccell; a switching relay arrangement connected in circuit with saidcapacitor and the external light source for automatically dischargingsaid capacitor when the same reaches a predetermined voltage, thedischarge current of said capacitor switching off the external lightsource; adjustable electrical control members connected in series withsaid capacitor; and a scalecalibr-ated in accordance with thelight-sensitivity and light-permeability of the copying paper arrangedon said casing in juxtaposition to said adjustable electrical controlmembers, said adjustable control members controlling the time period forcharging said capacitor to its predetermined voltage in accordance withthe characteristic of said copying paper as indicated on said scale,said casing, light-permeable support, photo-electric cell, capacitor andswitching relay arrangement forming a structural unit with the imageframe.

14. Copying apparatus in accordance with claim 13 wherein the controlmembers include additional correcting members, adjusting means coupledto said correcting members for changing the illumination time of thecopying paper by the external light source independently, said adjustingmeans being provided with resilient biasing members for returning saidadjusting means to an inoperative position when the same is not in use.

15. Copying apparatus in accordance with claim 13, wherein saidphoto-electric cell is arranged underneath said image frame between theinner edge thereof and one of the walls of said casing providing achamber beneath the copying paper, and a reflector is arranged in saidchamber for uniformly reflecting light passing through the copying papertowards said photo-electric cell.

16. An auxiliary apparatus for the optical copying of photographicnegatives, comprising, in combination, a casing; shiftable masks forselectively adjusting the size of the image frame on the copying paper;a photoelectric cell arranged in the casing so that light coming from aprojection light source and passing through the image frame of thecopying paper uniformly impinges on said photo-electric cell; acapacitor connected in circuit with said photo-electric cell so as to becharged by the current produced thereby, which current is a function ofthe mean brightness of the desired image frame; electrical controlapparatus connected in circuit with said capacitor and with theprojection light source for switching ofi the projection light sourceafter a time period determined by the amount said capacitor is chargedby the current produced in the photo-electric cell; and electricalcontrol members mechanically coupled to said electrical controlapparatus and said shiftable masks for changing the charging currentcircuit of said capacitor to automatically compensate for any change inthe size of said image frame defined by said shiftable masks.

1']. Auxiliary apparatus in accordance with claim 16 wherein saidphoto-electric cell and the electrical control members for the controlarrangement are arranged along the walls of the casing providing achamber beneath the copying paper, and a reflector arranged in saidchambers for reflecting light passing through the copying paper to wardssaid photo-electric cell.

18. In a device for the optical copying of photographic negatives, incombination, a casing; a light-permeable support for a light-sensitivepaper and an image frame arranged on top of said casing, said lightpermeable support defining a light-accessible area thereunder in saidcasing; a photo electric cell arranged within said casing outside saidarea; and light attenuating and directing means in said area fordifferently attenuating light passing through different portions of saidsupport and for directing the light to said photoelectric cell in such amanner that the mean intensity of light transmitted through saidlight-permeable support and directed to said photo electric cell issubstantially proportionate to the mean of the light intensities passingthrough different portions of said light permeable support into saidcasing and irrespective of the distance of said different portions fromsaid photoelectric cell.

19. A device as defined in claim 18 in which said light attenuating anddirecting means are constituted by reflector means positioned below saidlight-permeable support and having a plurality of reflecting elements ofvarying reflectivity arranged in such a manner that the mean intensityof light reflected onto said photoelectric cell is substantiallyproportional to the mean of the light intensities passing throughdifferent portions of said light permeable support into said casing andirrespective of the distance of said different portions from saidphotoelectric cell.

20. A device as defined in claim 18 in which said photoelectric cell isarranged within said casing outside said area and laterally offsettherefrom and in which said light attenuating and directing means areconstituted by curved reflector means positioned below saidlight-permeable support and extending arcuately from one area adjacentsaid light permeable support and more remote from said photoelectriccell to another area nearer said photoelectric cell and remote from saidlight permeable support and having a plurality of elements of varyingreflectivity arranged in such a manner that the overall reflectivity ofsaid reflector means decreases from said one area toward said otherarea.

21. In a device for the optical copying of photographic negatives, incombination, a casing; a light permeable support for a light-sensitivepaper and an image frame arranged on said casing; a photoelectric cellarranged within said casing so that the lengths of the paths of lightpassing through different portions of said light permeable support tosaid photoelectric cell are different; and light attenuating anddirecting means for differently attenuating li-ght passing throughdifferent portions of said support and for directing the light to saidphotoelectric cell in such a manner that, in the event equal amounts oflight pass through different portions of said support, the amounts oflight received by said photoelectric cell from said different portionswill be equal and irrespective of the length of the light path from therespective portion to said photoelectric cell.

References Cited in the file of this patent UNITED STATES PATENTS1,939,243 Twyrnan Dec. 12, 1933 1,953,471 Eich Apr. 3, 1934 2,165,965Gaebel et al July 11, 1939 2,484,299 Labrum Oct. 11, 1949 2,651,866Tuttle Sept. 15, 1953

